New concerns about the health affects of fine particles raise the possibility of a continuation of the trend towards more stringent emissions limits for the utility industry. The current belief is that ambient fine particles consist of both primary and secondary particles and that, in many regions of the country, SO.sub.2 emissions from point sources make a significant contribution to the formation of the secondary particulate matter. These concerns, together with the pressure to cut costs to survive in a competitive environment, point to the need for new, inexpensive technologies to control particulate and SO.sub.2 emissions from power plants.
Electric power generating plants, industrial boilers and other industrial processes generate particulates, acid gases and toxic materials that are harmful to the environment. Particulate matter can remain suspended in the air for an extended period and when breathed can present a potential health hazard. The particulates also tend to settle on surfaces such as buildings, machinery or curtains, where they can cause unsightly discoloration and interfere with proper function of equipment. In addition, trace metals are harmful to humans and other animal species. Thus, it is important to remove particulates from an exhaust gas stream. Moreover, acid gases, such as SO.sub.2 and SO.sub.3 have been found to contribute to damaging acid rain. Technologies for control of acid gases such as spray dryers and scrubbers are well known in the art. However, such control systems are expensive and their installation requires significant amounts of space.
There are a number of commercial technologies that can separately control power plant particulate and SO.sub.2 emissions. Both ESP (Electrostatic Precipitator) and fabric filters are used for particulate control, and either wet or dry scrubbers are used for SO.sub.2 control. The costs for both controls are moderate in terms of overall power generating costs, but both technologies require equipment that is substantial in size, and both technologies require considerable maintenance.
Prior Art Patents
Sparks et al in U.S. Pat. No. 4,885,139 teach an electrostatic precipitator for removing sulfur oxides and particulate matter by treating sulfur oxide or other acidic gases in a multi-stage electrostatic precipitator within a single housing. The sulfur oxides and acidic gas removal system works by spraying a neutralizing slurry or solution into incoming flue gas to form neutral salts which dry in a reaction zone. As pointed out in the patent to Sparks et al, the first stage of the electrostatic precipitator involves absorption of the SO.sub.x by the injected alkali drops; the second phase consists of SO.sub.x transfer to the wet alkali particles which exit after the drops are evaporated and the final phase consists of reaction of SO.sub.x with the dry particles. Note particularly that the flue gas is sprayed upstream from the collection plates.
Plaks et al in U.S. Pat. No. 5,601,791 teach an electrostatic precipitator for removal of acid gases by spraying an acid gas neutralizing agent through nozzles at a point upstream of the electrostatic collectors. In an alternative embodiment, the neutralizing agent is introduced into the transition zone which is upstream of the plates; and in a still further embodiment the neutralizing agent is injected at a point upstream of one set of plates and downstream of a second set of plates, that is, between two sets of plates in the stream. A careful review of the prior art patents indicates that none teaches an electrostatic precipitator where in the acid neutralization step takes place down stream of all the collection plates.
In addition, regarding the Sparks and Plaks patents, there are significant differences between the technology in these patents and the concept of the instant invention. In both of the patents, a slurry is sprayed into a precipitator, either in the transition piece just ahead of the precipitator (ESP) or in the front of the ESP in the volume that was occupied by the front or first field (first set of collection plates) of the ESP. In either case, the slurry must evaporate to dryness before it is collected in the ESP because the ESP is operated in a dry mode. The ESP must collect this particulate matter; and further, the ESP mixes the SO.sub.2 sorbent reaction products with the fly ash which may create a waste product that is difficult to dispose of. In a small ESP, the increased dust loading (and/or loss of the front field) may make it very difficult to maintain compliance with particulate emission limits. In addition, the spray system must produce very fine droplets if these droplets are to evaporate in the one or two second residence time between the spray nozzles and the first active field of the ESP.
In contrast, the new concept of this invention leaves the front part of the ESP alone and modifies the last one or two fields. Because of the exponential nature of the collection process, most of the fly ash is collected before reaching the part of the ESP that has been modified. Typically, 90% or more of the fly ash is collected in the unmodified ESP fields, and this uncontaminated ash can be gold as a concrete or cement additive. The last one or two fields are modified to operate in a wet mode, continuously eliminating the need to dry the sorbent before it is collected. This new approach actually improves the collection of particulate matter because it eliminates power limitations that can be imposed by the high electrical resistivity of dry fly ash and furthermore wet plate cleaning also eliminates reentrainment. In fact, EPRI-sponsored tests have demonstrated that converting the last field of a conventional dry ESP to wet operation could reduce outlet particulate emissions by a factor of five or more. Since in the new concept a clear liquid is sprayed into the ESP, there is no increase in the particulate matter the ESP must collect. Furthermore, the new process produces a waste product that is relatively easy to dispose of.
Objects of the Invention
It is an object of the invention to provide an apparatus and process for removing acidic gas and particulate matter from the gas stream of an ESP and to more efficiently collect and dispose of gaseous and particulate pollutants.
A further object of the invention is to provide an ESP that can remove acid gases, as well as toxic gases, without major modification of existing ESP equipment.
A still further object of the invention is to provide an ESP of high efficiency and durability.
A yet further object of this invention is to fashion an ESP improvement that is easy to retrofit into existing equipment.
An important object of this invention is to develop a device having improved efficiency for ESPs (Electrostatic precipitators) along with more reliable scrubber operation.
A significant object of this invention is to employ a clear liquor scrubbing solution which will minimize plugging and scaling of the equipment.
Another significant feature of this invention is the ability of the ESP to efficiently remove particulate matter.